The circuitry of protein tyrosine phosphorylation underlies many fundamental cellular processes, such as signal transduction, cell division, and growth control. The primary components of these circuits are protein tyrosine kinases (PTKs) and protein tyrosine phosphotases (PTPs). A broad outline of the complex interplay between these enzymes is understood, but the details of this relationship, particularly regarding the PM, are only beginning to emerge. The PTP gene family encodes two major classes of enzyme a large, transmembrane, receptor-type molecules and smaller, intracellular forms. While most attention has been devoted to the regulation and role of transmembrane, receptor PTPs, relatively little is known about their more abundant intracellular counterparts. TV goal of this study is to examine the mechanisms by which an abundant, prototypic intracellular PTP (PTP1B) is regulated and to explore its possible role as a tumor suppressor. Two preliminary observations hint at the mechanisms by which PTP1B may be regulated a) PTP1B is extraordinary sensitive to proteolysis, resulting in the generation of a smaller, more hydrophilic enzyme with different biological effects than its parent molecule; and, b) PTP1B undergoes cell-cycle specific phosphorylation. Using pulse-chase metabolic labeling, we will examine whether cells transiently transfected with PTP1B generate the truncated form, and, if so, if this form has a different specific activity than the full-length enzyme. To determine the functional consequences of the observed phosphorylation of PTP1B, we will examine whether the phosphorylated and unphosphorylated forms of PTP1B differ in cellular location and specific activity. We will also attempt to uncover the kinase(s) and phosphatase(s) that act upon PTP1B, as well as map the site(s) of phosphorylation. The possible role of PTP1B as a tumor suppressor will be explored using an steroid-inducible PTP expression system we have established in NIH-3T3 cells. When PTP1B expression is induced, these cells resist neoplastic transformation by v-src oncogene. We will examine PTP1B-expressing, v-src transfected NIH-3T3 cells for the loss of particular phosphotyrosyl proteins and/or expression of genes known to be induced by v-src. To determine whether PTP expression can revert an established malignancy, we will transfect a PTP expression vector into v-src transformed cells. We will also determine whether PTP1B expression prevents transformation by a variety of other PTK and non-PTK oncogenes. We anticipate that the insight we gain concerning the regulation and role of PTP1B will fill an important gap in our understanding of protein tyrosine phosphorylation. The results of the proposed experiments may also elucidate which key elements among the myriad changes in cellular physiology that occur as the result of PTK oncogene expression are required for the transformation process.